The invention relates to an apparatus and a method for determining the filling level of a product in a container with a transmission unit, which generates high-frequency signals and emits them at a predetermined transmission repetition frequency or shot rate (PRF=Pulse Repetition Frequency) in the direction of the surface of the filled product, the high-frequency signals being reflected at the surface of the product, with a receiving unit, which receives the reflected signals, with a delay circuit, which transforms the high-frequency signals/reflected signals into low-frequency signals in accordance with a predetermined translation factor, and with an evaluation unit, which determines the filling level of the product in the container on the basis of the delay time of the signals.
Apparatuses and methods for determining the filling level via the delay time of measuring signals (pulse or FMCW frequency-modulated continuous signals) use the physical law according to which the transit distance is equal to the product of the delay time and propagation velocity. In the case of filling level measurement, the transit distance corresponds to twice the distance between the antenna and the surface of the filled product. The actual useful echo signal and its delay time are preferably determined on the basis of the echo function or the digital envelope curve of the low-frequency signal, the echo curve reproducing the amplitudes of the echo signals as a function of the xe2x80x98antennaxe2x80x94surface of the filled productxe2x80x99 distance or the delay time. The filling level itself is then obtained from the difference between the known distance of the antenna from the bottom of the container and the distance of the surface of the filled product from the antenna, determined by the measurement.
DE 31 07 444 A1 provides a description of a high-resolution pulsed radar method. A generator generates first microwave pulses and transmits them via an antenna at a predetermined transmission repetition frequency in the direction of the surface of the filled product. A further generator generates reference microwave pulses, which are identical to the first microwave pulses but differ slightly from them in the transmission repetition frequency. The echo signal and the reference signal are mixed. At the output of the mixer there is an intermediate-frequency signal. The intermediate-frequency signal has the same waveform as the echo signal, but is stretched in comparison with the latter by a translation factor which is equal to a quotient of the transmission repetition frequency and the difference in frequency between the repetition frequency of the first microwave pulses and the repetition frequency of the reference microwave pulses. At a transmission repetition frequency of several megahertz, a difference in frequency of a few hertz and a microwave frequency of several gigahertz, the frequency of the intermediate-frequency signal is far below 1 MHz. The advantage of the transformation to the intermediate frequency is that relatively slow, and consequently low-cost, electronic components can be used for signal acquisition and/or signal evaluation. You are referred in this connection also to the German utility model DE-U 298 15 069. 7, which describes the known transformation technique in the case of a TDR filling-level measuring device.
A prerequisite for time delay by means of so-called sequential sampling is that the time difference between two successive sampling points is to a great extent constant. In the case of the mixer principle described above, preferably two quartz oscillators generate two oscillations with slightly different frequencies. The slight xe2x80x98detuningxe2x80x99 of the two oscillations causes a phase shift, increasing linearly with each period.
One of the disadvantages of the mixer principle is the relatively high power consumption, so that the energy supply by means of a 4-20 mA current loop, a widespread industrial standard, can be provided here only at measuring rates of one measured value per second.
An object of the invention is to provide a highly accurate and low-cost delay circuit. A further object of the invention is to provide a method with which a translation of the high-frequency signals into the low-frequency range can be carried out at low cost and with high accuracy. These objects are achieved by a delay circuit which has the following elements: a transmission oscillator, which generates transmission pulses at a transmission frequency; a sampling oscillator, which generates sampling pulses at a sampling frequency, the sampling frequency differing from the transmission frequency; a digital sampling circuit, which samples the transmission pulses with the sampling pulses; a closed-loop/open-loop control unit, which sets the difference in frequency between the transmission oscillator and the sampling oscillator in such a way that the predetermined translation factor is achieved. In particular, the digital sampling circuit is a phase detector, with the transmission frequency being sampled at the sampling frequency. According to the invention, consequently the period duration of the transmission frequency is plotted in a directly time-dilated form - and consequently transformed into the low-frequency range.